<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      body {
        margin: 0;
        display: flex;
        align-items: center;
        justify-content: center;
        height: 100vh;
      }
      .box{
        width: 200px;
        height: 200px;
        border-radius: 50%;
        background: deepskyblue;
        border: 20px solid rgba(200,200,200, .3);
        
        /* 初始值为border-box，背景会被元素的边框的外沿线裁切掉，这样透明边框无法正常显示
            将它设置为padding-box，浏览器就会使用内边距的外沿来把背景裁掉 */
        background-clip: padding-box; 
      }
    </style>
  </head>
  <body>
    <div class="box"></div>
  </body>
</html>
